1. Field of the Invention
The invention relates to an article comprising a slide gate plate with integral tube. The article is useful in the continuous casting of molten metals and finds particular utility with a tube exchanger mechanism.
2. Description of the Prior Art
In the continuous casting of steel, a stream of molten metal is typically directed from an aperture located at the bottom of a metallurgical vessel through a pour tube and into a second metallurgical vessel or mold. In a first step, the molten steel is directed from a ladle into a tundish through a ladle shroud. In a second step, the molten steel flows from the tundish through a sub-entry nozzle (SEN) or sub-entry shroud (SES) and into a mold. The flow rate of molten steel may be controlled by raising or lowering a stopper rod from a seating position at the aperture. A slide gate valve may also be used to control melt flow.
Pour tubes must be replaced periodically during the casting process. Changing a tube disrupts melt flow and can affect product yield or quality. A tube changer mechanism minimizes this disruption by quickly replacing a spent tube with a new tube. Such mechanisms typically use a hydraulic cylinder to push a spent pour tube out of a casting position and simultaneously drive a new pour tube into the casting position. Product quality is thereby improved because the disruption in the melt flow is reduced.
A tube changer mechanism typically has an upper fixed refractory plate and a lower sliding refractory plate, or tube plate, with an integral pour tube. A top surface of the tube plate compressively engages the lower surface of the upper fixed plate. The combination of the tube plate and pour tube into a single piece is referred to as a monoblock. Frequently, the monoblock is partially metal encased. The upper fixed plate may be connected to a nozzle extending through the aperture of the metallurgical vessel or the nozzle may be integral with the upper fixed plate. In either case, the upper fixed plate has an orifice which aligns with the bore of the nozzle. After exiting the metallurgical vessel and passing through the orifice, the melt stream enters a bore in the monoblock.
Both the upper fixed plate and the tube plate should be hard and scratch-resistant to minimize scoring, which may occur during the tube changing procedure. Scoring creates channels into which molten steel may penetrate. The steel can then solidify inside these channels and cause further damage to the plate surfaces. Scoring may also permit air aspiration into the molten metal. Air, particularly oxygen, has a detrimental effect on the quality of the solidified melt and can also cause clogging of the bore by precipitating oxides from the melt. To facilitate tube changing, a hard surface on the tube plate may be desirable to minimize scoring, break through any solidified steel, or remove precipitates which may have collected inside the bore.
Physical requirements for the pour tube are substantially different from the sliding plate. The pour tube experiences a rapid increase in temperature as molten metal quickly flows into the bore of a newly inserted tube. Thermal shock-resistance, not hardness, is therefore, critical to a pour tube. These conflicting design parameters for the tube plate and pour tube have caused manufacturers to design monoblocks using two or more different ceramic compositions. These two compositions often comprise two or more separate parts, which must then be united.
Various methods are available to join the tube plate and the pour tube. Commonly, a fired pour tube is set in a metal encasement and a fired refractory plate is cemented on top of the tube. This process requires additional manufacturing beyond the single firing step needed for the tube plate or pour tube alone. Additionally, this process creates a joint line between the tube plate and the pour tube. The joint may fail and permit air aspiration into the molten metal stream.
Alternatively, a hard ceramic insert may be cemented into a recess around the bore on the top surface of the tube plate. The body of the piece may thus be formed from a single thermal shock-resistant composition, most typically a carbon-bonded composition such as alumina graphite. The hard, scratch-resistant ceramic around the bore reduces scoring and removes precipitates found within the bore. For example, U.S. Pat. No. 5,335,833 teaches the use of a zirconia insert around the bore. Detrimentally, a joint line is created between the body of the piece and the insert. Such a joint will extend into the bore of the monoblock.
U.S. Pat. No. 5,348,202 succeeds in eliminating the joint in the bore by copressing the hard ceramic with the thermal shock-resistant ceramic. The hard ceramic is distributed on the top surface of the tube plate, most commonly either around the bore or across the entire top surface of the tube plate. This design is susceptible to radial cracking around the bore. It is well known that harder ceramic compositions are more sensitive to thermal shock than softer compositions. Faced with a huge thermal shock from resumption of the melt flow, the hard ceramic around the bore may experience a prodigious amount of radial cracking. Cracking creates avenues for air infiltration into the molten steel and leads to chipping, erosion and solidifying of the steel within the cracks. Geometric variations on this idea, including extending the hard ceramic down the bore, have not eliminated radial cracking. Horizontal cracking may also occur near the junction of the tube plate and the pour tube as the hard ceramic expands with temperature at a greater rate than the shock-resistant ceramic surrounding it.
An alternative solution exists in slide gate plates. UK Pat. Appl. No. 2,113,806A teaches using a softer material around the bore to control radial cracking and a hard material on the plate surface to control scoring. The softer material is present as an insert cemented into a recess of the hard material. Again, a joint remains which permits air aspiration into the molten metal stream.
A need persists for an integral refractory pouring assembly that overcomes the dual requirements of scratch-resistance at the tube plate surface and thermal shock-resistance in the tube and around the bore. Furthermore, the component's design should eliminate joints within the bore, radial cracking around the bore, and horizontal cracking near the tube plate/pour tube junction.